OpenAI exception handling and httpx dependency

[marklysze]

Why are these changes needed?

Further updates now that openai is an optional dependency (based on tests failing):
OpenAI exception handling and httpx dependency

Related issue number

N/A

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
autogen/oai/client.py	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
autogen/oai/client.py	50.07% <0.00%> (+6.71%)	⬆️

... and 83 files with indirect coverage changes